1. Field of the Invention
The invention relates to a vortex flowmeter having a medium chamber that can have a medium at least partially flowing through it and is defined by a device wall, at least one bluff body provided in the medium chamber and at least one pressure sensor provided in the effective range of the bluff body, wherein the deflection of the pressure sensor is used to metrologically detect the pressure in the medium next to the pressure sensor, wherein at least one optical fiber is arranged in and/or on the pressure sensor for measuring the deflection of the pressure sensor and wherein the optical fiber is guided from the medium chamber through a pressure-resistant fiber duct in the device wall into medium-free surroundings. Furthermore, the invention also relates to such a fiber duct.
2. Description of Related Art
Vortex flowmeters have been known for a long time, wherein the measuring principle is based on the fact that, in a fluid or gaseous medium, a vortex street is formed behind a bluff body that has a medium passing it, the vortex street being fowled by vortices stripped away from the bluff body moving along with the current. The frequency with which the vortices are stripped away from the bluff body is dependent on the velocity of the current, wherein this correlation is nearly linear under certain conditions. In any case, the measurement of the vortex frequency presents a suitable means for determining the velocity of the current of the medium, which is why it is indirectly—under additional consideration of, for example, pressure and temperature—possible to determine the volume and mass flow using the vortex frequency measurement. The vortices of the medium occurring in a vortex street lead to local pressure fluctuations that can be detected by pressure sensors. Such a pressure sensor can have an essentially level measuring diaphragm and has to be arranged in the vortex street so that the vortices created by the bluff body pass by the measuring diaphragm of the pressure sensor at least indirectly and, thus, can be detected. To this end, the pressure sensor can be provided downstream, behind the bluff body, but can also be formed in the actual bluff body or, for example, can be arranged above the bluff body, when the pressure sensor, for example, indirectly determines the pressure fluctuations of the vortex street via channels in the housing of the flowmeter.
Very different methods are known from the prior art for determining the deflection of the pressure sensor, often capacitive or inductive effects are used, and sometimes piezoceramics are used. It is also known from the prior art to use optical fibers for determining the movement of the pressure sensor, wherein, for example, designs are known in which the optical fiber runs practically perpendicular to the front of the measuring diaphragm of the pressure sensor and the diaphragm is supplied with light on its end face, this light being reflected by the measuring diaphragm and subsequently used for the detection of movement. Vortex flowmeters are also known from the prior art in which an optical fiber lies against the pressure sensor, wherein the optical fiber is deflected together with the pressure sensor when the sensor is subject to pressure or differential pressure, with the result that the optical fiber is stretched and/or compressed, i.e., the optical fiber experiences a change in length. Such a change in length can be optically evaluated, as is known, with great precision, for example, using a known method that is based on the interference of electromagnetic waves. Using this method, it is easily possible to reliably detect changes in length in a range of the wavelength of the electromagnetic waves used.
Optical fibers are comparably sensitive, in particular in view of bending and buckling, so that it is a particular challenge to provide a suitable fiber duct in the device wall for feeding the fiber from the medium chamber into the medium-free surroundings. Extreme conditions can prevail in the medium chamber in view of pressure (hundreds of bar), in view of temperature (hundreds of ° C.) and also in view of chemical aggressiveness of the medium. The fiber duct, thus, has to be suitable for withstanding these conditions, so that the medium-free surroundings, in which, for example, the evaluation electronics for the signal of the optical fiber are located, are reliably sealed off from the medium chamber.
It is known from the prior art to glue or to solder an optical fiber with a mechanically stable sealing element before installation, wherein in the latter case, metalized optical fibers have to be used (e.g., UK Patent GB 2 089 065 or also U.S. Pat. No. 3,825,320). This course of action is comparably complex since the optical fibers are usually not prepared by the producers of vortex flowmeters, rather have to be prepared with a suitable glued or soldered sealing element by a third party. Furthermore, optical fibers provided with such sealing elements cannot be flexibly used in view of their dimensions, since an optical fiber normally extends into the medium chamber of a vortex flowmeter in a looped manner and, in this respect, has to be provided with sealing elements on both ends of the fiber, so that a certain distance from sealing element to sealing element results on the optical fiber for every device variation and structural changes in view of positioning the pressure sensor are not possible when maintaining the sealing element distance. Furthermore, the installation of such fiber ducts is also mechanically comparably complex; the pressure sensor cannot be dismantled without removing the fiber duct from the device wall. The lack of flexibility in the length of the optical fibers produced in this manner also often leads to the occurrence of additional splice sites in the medium-free surroundings.